In knee arthroplasty, portions of the natural knee joint are replaced with prosthetic knee components. Typically, these components include a tibial component, a femoral component, and a patellar component. The femoral component generally includes a pair of spaced condyles that articulate with the tibial component. The components are made of materials that exhibit a low coefficient of friction when they articulate against one another.
When the articulating ends of both the femur and tibia are replaced, the procedure is referred to as total knee replacement or TKR. Much effort has been devoted to performing a TKR that restores normal, pain-free, functions of the knee in a short period of postoperative time.
Several factors lead to long-term success of TKR. One important factor is soft-tissue balancing. The normal, non-diseased knee is considered properly balanced when the deflection between the medial and lateral condyles and the tibial plateau is equal throughout the entire range of motion. If this balance is not achieved, abnormal knee kinematics occurs, and the TKR components and surrounding soft tissue can experience excessive forces even during normal range of motion. These excessive forces can further cause an abnormal gait, pain, and early failure of total knee replacements.
Soft-tissue balancing can be achieved in TKR if the components are correctly sized and properly placed. In order to achieve proper placement during a TKR surgery, equal tibial-femoral flexion gaps and extension gaps must be achieved. The flexion gap is defined as the space between the posterior coronal cut on the distal femur and transverse cut on the proximal tibia, while the knee is in 90 degrees of flexion. The extension gap is defined as the space between the transverse cut on the distal femur and the transverse proximal tibial cut while the knee is in complete extension. Soft tissue balance occurs when stability is achieved in both flexion and extension.
During a TKR surgery, a series of surgical compromises is often used to achieve a balance of flexion and extension gaps. Elevation of the joint line is one such compromise. An elevation of the joint line occurs when there is a change in distance from the original articular surface to the newly reconstructed surface. This change in distance is typically measured as a vertical distance from a fixed point on the tibia.
For several reasons, the joint line can become elevated. Excessive medial or lateral releases and insertion of thicker plastic inserts can cause the line to elevate. Further, the joint line can become elevated when the femoral component is undersized. Such an undersize can create a larger flexion gap than extension gap. To balance these gaps, more bone may need to be removed from the distal femur; and this additional bone loss raises the joint line.
Unfortunately, a change in the joint line can negatively affect a wide array of components and operations of the knee, such as the functions of the PCL, collateral ligaments, and patello-femoral joint mechanics. These problems can be avoided or minimized if elevation of the joint line is reduced or, better yet, eliminated.
Another surgical compromise often occurs when soft tissue gaps are not balanced when implanting a distal femoral knee prosthesis. A healthy balance of these gaps maintains the natural kinematics of the patient. The compromise occurs when the operating surgeon must choose one of six or seven differently sized distal femur prostheses; and the size of these prostheses may not exactly match the size of an ideal prosthesis for the patient. For example, current anterior referencing methodology to achieve balanced flexion and extension gaps in most patients requires the surgeon to slightly alter the joint line. If an anterior referencing sizing guide falls between two sizes, the surgeon could be forced to choose a smaller size prosthesis so the flexion gap is not overstuffed. A smaller prosthesis, in such an instance however, can consequently enlarge the flexion gap as much as 3.5 mm to 4 mm.
Another important factor that contributes to the long-term success of total knee replacements is loading and kinematics of the patellar-femoral joint. Complications associated with patella failures account for up to 50% of TKR revision procedures. Many of these complications occur because of improper or unnatural loading or kinematics of the patellar-femoral joint. For example, overstuffing the patellar-femoral joint is one major cause of improper soft tissue loading and kinematics. In this regard, many surgeons use posterior referencing instrumentation when sizing and preparing the femur for implant resurfacing. On the one hand, posterior referencing allows the surgeon to balance the tibial-femoral flexion and extension gaps without necessarily changing the joint line. On the other hand though, the use of posterior referencing increases the risk of notching the anterior cortex and overstuffing the patellar-femoral joint.
In short, current knee systems often require an unwanted surgical compromise. Such compromises can alter the natural joint line of the patient or overstuff the patellar-femoral joint. Unfortunately, these compromises also negatively affect the natural kinematics of the patient and can, for example, increase strain on the PCL and other tendons and ligaments, increase implant wear, and decrease implant function. Patients may be more likely to experience pain, reduced function, and more frequent revision surgeries.
Current knee systems have other disadvantages as well. Distal femoral prostheses are simply too large to fit through small incisions that are used during a minimally invasive surgery or MIS. MIS has many advantages over traditional surgical techniques since it provides shorter incisions, faster recovery times, and generally less pain for the patient. The surgical technique, though, has limitations. As noted, current tricompartmental distal femoral prostheses cannot fit through the small incision, usually three inches in length. To date, MIS has been generally limited to unicondylar or unicompartmental knee replacement prostheses that are much smaller in size and able to fit through the incision.
It would be advantageous to have a modular knee prosthetic system that has advantages over prior knee prosthetic systems and techniques. Such a system would have greater modular versatility to accommodate different patient anatomies and joint conditions while maintaining a relatively low component count.